Enhancement of mechanical performance and heavy metal immobilization in red mud-based cementitious materials through sodium silicate optimization and CaO modification.

Red mud (RM), a highly alkaline by-product of alumina production, poses serious environmental risks, while its high-value utilization remains challenging. Although red mud-based cementitious materials have been widely investigated, simultaneously achieving high mechanical performance and effective heavy metal immobilization remains difficult because the coupling mechanism between hydration evolution and contaminant stabilization is still unclear. This study developed an optimized red mud-based cementitious material to simultaneously enhance compressive strength and heavy metal stabilization while elucidating the underlying mechanism. The final optimized formulation achieved a 28-day compressive strength of 9.02 MPa while maintaining acceptable flowability. At the final 56-day test point, the leaching concentrations of Pb, As, Cd, and Mn were more than 98% lower than those of the freshly prepared reference mixture. XRD and SEM observations were consistent with AFt formation, the development of poorly crystalline gel-like products, and progressive matrix densification. These structural changes were consistent with the observed decrease in heavy metal leaching. However, the chemical states and binding environments of the immobilized metals were not directly determined. These findings suggest a link between hydration-driven microstructural evolution, strength development, and contaminant stabilization, supporting the sustainable use of red mud in low-carbon cementitious materials.